As per the observations (Figure 4-8), variation of PTI could be clearly defined with the travel time variation pattern within the period 3.00pm – 7.00pm, which is evening peak time. When congestion onset, PTI starts to increase from 3.00pm and rises up to maximum value of 6.92 at 5.15pm. Then it starts to drop down and reaches to an average value of 1.14 with free flow condition around 7.00pm. These results indicate that the Coronation Drive is highly congested and unreliable in evening peak time for inbound traffic.
Variation of PTI – Recurrent Working Days
Table 4-3 provides the maximum values of PTI experienced in the evening peak time from 3.00pm to 6.45pm for All Working Days and Recurrent Working Days respectively. For Recurrent Working Days, the variation pattern of PTI is approximately similar to that of All Working Days up to 3.00pm, but the peak time starts at 4.00pm and PTI reaches to 3.77 at 5.15pm. This value is 3.15 less than that of All Working Days. This difference of PTI indicates that there‟s a considerable impact of incidents on average travel time.
Figure 4-8: Variation of Planning Time Index – Evening Peak Time
Table 4-3: Variation of Planning Time Index – Evening Peak Time
Time All Working Days Recurrent Working Days
3.00pm 1.52 1.33
5.15pm 6.92 3.77
6.45pm 1.74 1.18
Table 4-3 provides the maximum values of PTI experienced in the evening peak time from 3.00pm to 6.45pm for All Working Days and Recurrent Working
Days respectively.
Variation of Cv – All Working Days:
4.3.6
As per the observations, and as illustrated in Figure 4 - 9, Cv rises up to 0.3 in
the morning peak time at 8.45am. Then it starts to decrease and reach to a value of 0.1 at 10.00am. When there‟s free flow condition, Cv varies between 0.10 – 0.07 up
to maximum value of 0.77 at 6.00pm. Then it starts to drop down and reaches to an average value of 0.11 around 8.00pm.
Variation of Cv – Recurrent Working Days
As illustrated in Figure 4 - 9, the variation pattern of Cv is similar to that of All
Working Days in the morning. But at the evening peak time, the value of Cv reaches
to 0.43 at 5.00pm., which is 0.34 less than that of All Working Days. This difference of Cv indicates that there‟s a considerable impact of incidents on the variation of
average travel time.
Figure 4-9: Variation of Coefficient of Variation (Cv)
Table 4-4: Variation of Coefficient of Variation (Cv) – Evening Peak Time
Time All Working Days Recurrent Working Days
2.00pm 0.07 0.07
5.00pm 0.71 0.43
6.00pm 0.77 0.40
Table 4-4 provides the maximum values of Cv experienced in the evening peak
time from 2.00pm to 8.00pm for All Working Days and Recurrent Working Days respectively.
4.3.7 Comparison of Reliability measures for Coronation Drive
This study evaluated the Planning Time Index (PTI), Buffer Time Index (BTI), Travel Time Index (TTI) and Coefficient of Variation (Cv). The variation patterns of different indicators for All Working Days are depicted in Figure (4-10). The results indicate that the different reliability indicators exhibit the similar trend with the increasing traffic congestion, and the traffic congestion and degree of impact of incidents directly related to the travel time reliability.
As per the results, the highest values observed for PTI, BTI, TTI and Cv were 6.92, 1.78, 2.80 and 0.77 respectively, which recorded around 5.30pm. A good correlation between four reliability measures could be observed all the day. The correlation coefficients between different indicators are provided in table 4-5.
Table 4-5: Correlation Coefficients Reliability Indicators Correlation Coefficients PTI - Cv 0.72 BTI - Cv 0.90 PTI - TTI 0.88 PTI - BTI 0.84 4.4 DISCUSSIONS
In this study, four reliability indicators were evaluated to analyse the travel time reliabilityindicators on selected road corridor, and incident records were used to examine the inpact of incidents on travel time reliability.
According to the results obtained for Coronation Drive, following observations were made.
- Coronation Drive is congested for inbound traffic, both in the morning and the evening peak periods. Analysing the morning peak time, it is observed this corridor experiences a regular congestion in the morning and it is more reliable compared to the high congestion in the evening peak period.
- As per the observations, evening peak period is highly congested and significant travel time variations were recorded resulting unreliability in travel time.
- The travel time reliability and the impact of incidents on travel time reliability with the variation pattern of buffer time, BTI, and PTI could be clearly observed and defined using the BMS data obtained.
- The increase of BTI due to incidents is nearly 60%. This indicates that there‟s a significant impact of incidents on travel time reliability, and the reduction in incidents and its durations will not only reduce congestion, but also has the potential to enhance its reliability.
- PTI at free flow condition varies between 1.10 – 1.55. This rises upto 6.92 at high cogestion. This indicates, at high congestion, commuters have to plan nearly 5 times on there trips than travel time at free flow condition.
- Cv varies exactly in the same pattern up to the start of evening peak time, in
both catogories indicating high travel time reliability. (All Working Days and
Recurrent Working Days) Then it starts to increase rapidly after 2.00pm, and
the variation pattern becomes different. In the analysis of All Working Days, it rises upto 0.77, and for Recurrent Working Days, the maximum value is 0.40. This difference of Cv indicates the considerable impact of incidents on
travel time reliability.
- The selected reliability indicators (BTI, TTI, PTI & Cv) represent the travel time reliability of Coronation Drive well, demonstrating a good correlation between them. This indicates that only few indicators are necessary for studying the travel time reliability. Table 4-6 compares the indicators for All
Working Days and Recurrent Working Days. It is observed that Cv has the
highest difference in the values. This indicates that Cv should be a better indicator to quantify the non-reliability due to incidents.
Table 4-6: Percentage increase of reliability measures due to incidents
Reliability Indicator All Working Days Recurrent Working Days Percentage Increased BTI 1.78 1.11 60.36% PTI 6.92 3.77 83.55% Cv 0.77 0.40 92.50% TTI 2.80 2.18 28.44%